1. Field of Invention
The invention relates to fingerprint imaging and specifically to a readout sequence and array shape that minimizes the data needed to reconstruct an image.
2. Prior Art
Fingerprint scanners are one form of biometric verification used to access data and devices. Devices that have potential use for biometric verification include laptop computers, personal digital assistants, cellular or mobile telephone handsets, and any other device requiring access restriction or security.
Fingerprint sensing technology is well known. One method uses an alternating current excitation signal that propagates an electric field through the finger while a capacitive sensor detects the ridges and valleys in the living layer of skin beneath the skin surface. Edwards, U.S. Pat. No. 4,429,413, Jan. 31, 1984, “Fingerprint sensor” discloses an integrated circuit for transforming the topological pattern of a finger into an electric output signal.
To reduce the array size and therefore cost, instead of providing a large square array capable of capturing the entire fingerprint, the finger is manually swiped across a wide and short rectangular sensor. Consecutive images are captured as the fingerprint passes across the sensor and are combined to form a full image of the fingerprint. A fingerprint scanner is typically a two-dimensional sensor array with a pixel width count much larger than the pixel height count. Pixel dimension of 128 by 8 or 12 and 192 by 16 are common, and other pixel sizes and aspect ratios can be used.
Standard fingerprint scanners interface to a host device using a specialized signal interface intended for the scanner or a general purpose interface such as universal serial bus (USB) See Lei et al., U.S. patent application publication US 2006/0069826 published Mar. 30, 2006 for an example of a fingerprint sensor that interfaces to an external computing device over USB.
A specialized interface for the sensor precludes compatibility with existing devices that are not designed with a physical layer interface suitable for connection to the sensor. A USB interface in a fingerprint sensor requires significant die area on the device and thereby increases cost of the sensor.
Fingerprint scanners have special mechanical and electrical needs due to the exposure of the active sensor area. The sensor must be exposed for the finger to contact with the sensor. Electrostatic discharge (ESD) protection must be provided on the sensor chip, which uses die area around the active sensor region.
Reduction of the number of interface signals is highly desirable. Each input or output (I/O) pin requires a bond wire pad on the integrated circuit, which uses die area and thus increases cost. Additionally, mechanical clearance distances are required between the bonding pads and the active sensor array. Reduction in the number of pads reduces clearance requirements and reduces die size.
An interface to standard processor devices is desirable to increase compatibility and avoid redesign of existing processors. A low cost sensor is desirable, which requires minimizing die area and I/O pad count.
Benckert, US 20020012455A1 (same as WO 01/87159 A1, issued as EP 1284651 B1), “Composite image generating method, and a fingerprint detection apparatus”, published 31 Jan. 2002, and incorporated herein by reference, discloses a fingerprint sensor and method of combining a sequence of partially overlapping fingerprint frames in real-time. The frames, described as 500 dpi grayscale images from a sensor measuring 1.5 mm by 14 mm (30 by 280 pixels) are transmitted to the data processor in a mobile telephone that then produces a composite fingerprint image. Correlation between the partially overlapping frames is used to determine the displacement vector between the frames that is used to assemble the frames. Sub-areas are correlated to reduce computational power.
FIG. 1 shows a prior art fingerprint sensor array configuration, comprised of a two-dimensional array of pixels and shown as a number of rows of pixels forming an image frame. As the finger is swiped across the fingerprint sensor, the series of image frames sensed by the imaging array must be transmitted to the host over an interface that has sufficient bandwidth to carry the pixel data. If the bandwidth of the interface is not sufficient, image data may be lost. Minimizing the data rate of the sensor output is desirable to reduce power consumption of the device and enable the device to interface over low speed connections.
Mobile telephone handsets and other devices commonly incorporate an analog to digital converter (ADC or A/D) used to monitor signals within the device. Additionally, such devices may include a low data rate digital interface. It is desirable to use an existing interface on a host device to avoid redesign of the host device. The bandwidth of the analog interface is generally limited due to its typical use sensing low bandwidth signals. To support a natural rate of finger swiping, the sensor must transmit an image at a sufficiently high image frame rate, which produces a bandwidth or data rate that is the product of the frame rate and frame size in pixels. The bandwidth required for conventional sensors may exceed the capability of a typical analog or digital interface. Successive frames of fingerprint images must be combined together to reconstruct a full 2-dimensional image. The reconstruction requires that some rows of each image frame are redundant so that the frames can be aligned for the reconstruction. The extent of overlap in the images can be range from 50% or higher or lower. In a 16-line image, thus 8 lines would overlap between successive frames. The reconstruction algorithms use the lines of overlap to combine the frames. Too few lines of overlap decrease the quality of the reconstructed image. The overlapping lines are redundant information and increase the total data output from the sensor.
The prior art does not teach a solution to the objective of limiting the bandwidth of the image data to match available interfaces. A reduction in the bandwidth or data rate of the signal output from the fingerprint sensor is desirable.